A pneumatic switching valve is known from German Patent No. 41 20 824 C1 corresponding to European Patent No. EP 0 520 148 B1. The known switching valve includes a control piston having a cam surface with which the tappet of a locking valve may be actuated by axial displacement of the control piston. In this way, it is possible to switch from the stop position to reach the driving position and vice versa when driving the vehicle. Cam discs are axially displaceable together with the control piston, but they are coupled thereto to be commonly rotated. The cam discs include cam surfaces for actuating an inlet valve and an outlet valve. There are three valves—namely an inlet valve, an outlet valve and a locking valve—for each air suspension circuit. As another component, a bush is associated with the control piston, the bush being fixedly connected to the control piston and to the actuation element to be commonly rotated therewith. The arrangement of the bush and of the cam disc for the inlet valves and for the outlet valves have the effect that the design and construction of the known switching valve are rather complicated. Furthermore, there is a separate spring unit having an effect on the bush and on the control piston by the bush to cause an automatic returning movement from the lifting position (or from the lowering position) to reach the stop position as soon as the actuation element is released by the operator, as it is common after having finished a lifting process or a lowering process. The spring unit also makes the structure more complicated. The axes of the three valves serving for an air suspension circuit are located in one common plane intersecting with the axis of the switching valve and of the switching piston, respectively. In this way, the groups of valves for a plurality of air suspension circuits may be distributed about the circumference. Furthermore, there is a locking unit for preventing rotational movement of the switching piston in the driving position. The control piston is arranged in the housing of the switching valve to be axially movable by compressed air—especially by pulses of compressed air—to ensure that the driving position of the switching valve is maintained during driving the vehicle. It is a disadvantage of the known switching valve that the spring unit for returning the control piston to reach the stop position is rather complicated, and that the control piston has the described two-part design including the switching shaft and the bush. Additionally, realizing the cam surfaces at separate individual cam discs being arranged not to be rotatable and axially displaceable on the control piston is a substantial structural expenditure.
Another pneumatic switching valve is known from German Patent No. 26 23 235 C3. The known control unit is designed to be manually operable, and it includes a mechanical actuation element including a switching shaft and an actuation lever. The switching shaft may be pivoted about its axis by the actuation lever to reach five different angle positions including the stop position twice. The known switching valve includes cam discs being located on the switching shaft, the cam discs for each air suspension circuit actuating an inlet valve, an outlet valve and a locking valve. These three valves are arranged in a disc-like housing. The axes of the valves are located in one common plane perpendicular to the axis of the switching valve. In this way, the valves may also have a structure to serve for two or more air suspension circuits in an easy way. There are special catching units for the five positions such that partly automatic actuation is not possible. Proper actuation depends on the consciousness of the person actuating the switching valve. In this way, it is possible that the valve after having finished the loading operation does not automatically take the driving position.
Another switching valve is known from German Patent Application No. 25 10 954 A1. The switching valve has the positions “lifting”, “lowering”, “driving” and “stop”. The valves being associated with the separate air suspension circuits are not designed to include tappets, but they instead have a valve stem design. The known switching valve includes a separate control piston which may be subjected to compressed air such that the switching valve may take the driving position due to the compressed air. The two stems of the valve with their axes are designed to be parallel with respect to each other. There are return springs for the stem elements, the springs serving to return the switching valve to reach the stop position as soon as the mechanical actuation element is released.